1. Field of the Invention
The present invention relates to a scroll compressor installed in an air conditioner, a refrigerator, or the like, and in particular, relates to the shape of a scroll member.
2. Description of Related Art
FIG. 6 shows a cross-sectional view of a scroll compressor which is conventionally used. The scroll compressor comprises housing 6, fixed scroll member 1 which is fixed in housing 6, and orbiting scroll member 2 which is provided in housing 6 so as to freely rotate therein. Front case 5 which supports the orbital movement of orbiting scroll member 2 is fixed at an opening end side of housing 6, and shaft 7 which operates orbiting scroll member 2 so as to rotate is provided in front case 5. In shaft 7, crank pin 7a having axis X2 which is offset from axis X1 of shaft 7 is provided. This crank pin 7a is connected to boss 2c which is formed in the center of orbiting scroll member 2.
Fixed scroll member 1 is composed of fixed end plate (end plate) 1a and spiral wall body 1b. Orbiting scroll member 2 is composed of orbiting end plate (end plate) 2a and spiral wall body 2b. Spiral wall body 2b of orbiting scroll member 2 is assembled to spiral wall 1b of fixed scroll member 1, out of phase by 180 degrees, with spiral wall bodies 1b and 2b engaged with each other. Orbiting scroll member 2 orbitally moves with respect to fixed scroll member 1 via shaft 7. Accordingly, a compression chamber is formed between spiral wall bodies 1b and 2b. The volume of the compression chamber is gradually reduced by this orbital movement so that fluid in the compression chamber is compressed. The compressed high pressure fluid is ultimately discharged from discharge port 1c which is provided in the center of fixed end plate 1a. 
In the above-described scroll compressor, the volume of the compression chamber, which is a crescent-shaped airtight space formed at the outermost portion by both scroll members 1 and 2, is the volume of the fluid to be taken in, and the volume is gradually compressed. In order to increase the amount of the fluid to be taken in, that is, the volume to be compressed, it is required that the number of windings of each of spiral wall bodies 1b and 2b is increased or the height of each of spiral wall bodies 1b and 2b be increased. However, if the height of each of spiral wall bodies 1b and 2b be increased, there is a problem in that the rigidity of spiral wall bodies 1b and 2b against the compression reaction force of the fluid decreases.
In order to solve the above problem, the following construction is disclosed in Japanese Patent No. 1296413. FIGS. 7A and 7B are perspective views of fixed scroll member 1 and orbiting scroll member 2 proposed in Japanese Patent No. 1296413.
Fixed scroll member 1 is composed of fixed end plate 1a and spiral wall body 1b which is erected on a side surface of this fixed end plate 1a. This fixed end plate 1a is formed so as to correspond to the height of spiral wall body 2b of orbiting scroll member 2 to engage with a bottom portion by spiral wall body 1b which is composed of shallow bottom portion 1d (high site), which becomes high at the center side, and deep bottom portion 1e (low site), which becomes low at the outer peripheral end side.
Furthermore, orbiting scroll member 2 is composed of orbiting end plate 2a and spiral wall body 2b which is erected on a side surface of this orbiting end plate 2a. This orbiting end plate 2a is formed so as to correspond to the height of spiral wall body 1b of fixed scroll member 1 to engage with a bottom part of spiral wall body 2b which is composed of shallow bottom portion 2d (high site), which becomes high at the center side, and deep bottom portion 2e (low site), which becomes low at the outer peripheral end side.
At a side surface of each of end plates 1a and 2a of fixed scroll member 1 and orbiting scroll member 2, bottom side step portion 3 (step portion), which is high at the center portion and low at the outer peripheral end side, is formed. Additionally, corresponding to bottom side step portion 3 of each of end plates 1a and 2a, wall body side step portion 4 (step portion), which is low at the center portion and high at the outer peripheral end side, is formed on the spiral top edge of each of spiral wall bodies 1b and 2b. 
As a result, bottom side step portion 3 of fixed scroll member 1 is engaged with wall body side step portion 4 of orbiting scroll member 2, and bottom side step portion 3 of orbiting scroll member 2 is engaged with wall body side step portion 4 of fixed scroll member 1. When orbiting scroll member 2 orbitally moves, wall body side step portion 4 provided on each of spiral wall bodies 1b and 2b slides along a circular arc of bottom side step portion 3 formed on each of end plates 1a and 2a. 
In scroll members 1 and 2 formed as described above, since the height of the compression chamber of the outer peripheral side is large, the outside diameter of the scroll compressor is not increased and, at the same time, the amount of the fluid to be incorporated can be increased. Furthermore, since the height of the compression chamber of the center side is small, the volume of the compression chamber is decreased and, at the same time, the rigidity of the wall bodies is improved.
In the scroll compressor having a structure such as described above, orbiting scroll member 2 undergoes various operations when compression is performed. These operations are explained with reference to FIG. 8. In FIG. 8, shaft 7 (shown in FIG. 6) and crank pin 7a (shown in FIG. 6) are not shown.
As shown in FIG. 8, thrust direction gas force Fth and transverse gas force Fg due to the pressure of compression gas which is a fluid, and scroll driving force Fd due to crank pin 7a of shaft 7 acts on orbiting scroll member 2.
In other words, thrust direction gas force Fth is a force drawing orbiting scroll member 2 from fixed scroll member 1 along the direction of axis X1 (shown in FIG. 6) by gas pressure in the compression chamber. Additionally, transverse gas force Fg is a force drawing each of spiral wall bodies 1b and 2b along a transverse direction perpendicular to axis X1 by has pressure in the compression chamber. Furthermore, scroll driving force Fd is a rotational driving force added to boss 2c by crank pin 7a which rotates around axis X1 when shaft 7 rotates. Moreover, thrust force Fth is borne by an inside end surface of front case 5 on which orbiting scroll member 2 slides.
In the scroll compressor shown in FIG. 8, in order to obtain smooth orbital movement of orbiting scroll member 2, a predetermined clearance 6 (hereinafter, called xe2x80x9ctip clearancexe2x80x9d) is provided between the end of spiral wall body 2b of orbiting scroll member 2 and fixed end plate 1a of fixed scroll member 1.
By providing tip clearance xcex4, smooth orbital movement of orbiting scroll member 2 is ensured and resistance to thermal expansion by heat during the process of producing high pressure fluid in scroll members 1 and 2 is also ensured. However, there are problems related to this which are explained below.
As described above, among the forces acting on orbiting scroll member 2, as shown in FIG. 8, scroll driving force Fd and transverse gas force Fg act in opposite directions with respect to each other. As a result, moment M is produced which tends to overturn orbiting scroll member 2 or acts so that orbiting scroll member 2 becomes inclined. Furthermore, orbiting scroll member 2 tends to incline or overturn just by the present of tip clearance xcex4. In this case, the upper edge of orbiting scroll member 2 exerts pressure force F against fixed end plate 1a of fixed scroll member 1.
FIG. 9 is an enlarged side cross-sectional view of this state as seen from the side surface of wall body side step portion 4 of spiral wall body 2b. Orbiting scroll member 2 overturned during orbital movement makes point contact or line contact with deep bottom portion 1e which is a surface of fixed end plate 1a of fixed scroll member 1 at angle A of the convex side end of wall body side step portion 4 formed on spiral wall body 2b. This causes a power loss in the rotational drive force and abrasion of deep bottom portion 1e and spiral wall body 2b of orbiting scroll member 2.
In view of the above problems, it is an object of the present invention to provide a highly reliable scroll compressor which can reduce power loss due to the overturning of an orbiting scroll member and reduce the abrasion of parts.
In order to achieve the above object, the scroll compressor of the present invention has the following constitution.
The present invention is a scroll compressor comprising: a fixed scroll member which has a spiral wall body erected on a side surface of an end plate and which is fixed at a predetermined position; an orbiting scroll member which has a spiral wall body erected on a side surface of an end plate and which is supported so as to be orbitally movable while being prevented from rotating on its own axis, with the pair of spiral wall bodies engaged with each other; and a step portion provided on an upper edge of each spiral wall body in which a height between an upper surface of a bottom portion and the upper edge is low at a center side in a spiral direction and high at an outer peripheral end side, wherein a convex side end of at least one step portion is formed lower than an extrapolated line of the upper edge.
According to the above construction, even if the orbiting scroll member during orbital movement is overturned due to the presence of a tip clearance, the convex side end of the step portion of the spiral wall body does not strongly press against a surface of the end plate of the fixed scroll member, which is opposite the convex side end.
Furthermore, in the similarly formed step portion of the fixed scroll member, the convex side end of the step portion of the spiral wall body of the fixed scroll member does not strongly press against the surface of the end plate of the orbiting scroll member, which is opposite the convex side end.
According to the above construction, since at least one step portion of each scroll member is formed lower than an extrapolated line of the upper edge of the spiral wall body, the scroll members do not make contact with or press against each other when the scroll compressor is operated, therefore abrasion is prevented. Accordingly, a reliable scroll compressor which reduces power loss due to the overturning of an orbiting scroll member and which has a high efficiency is possible.
Furthermore, in the above scroll compressor, the convex side end of at least one of the step portions may have a chamfered shape or a rounded shape.
According to the above construction, even if the orbiting scroll member is overturned due to the presence of a tip clearance during orbital movement, the convex side end of the step portion is not scratched by sliding or does not press against the surface of the end plate, which is opposite to the convex side end. This convex side end is simply formed by removing a 45xc2x0 angle from the end of the convex side end or rounding the end of the convex side end. Furthermore, if this convex side end is formed on the step portion of the fixed scroll member, the same shape and the same effects are obtained.
Furthermore, since this convex side end is simply formed, the manufacturing cost is decreased. Moreover, the scroll members do not make contact with or press against each other when the scroll compressor is operated, therefore, a reliable scroll compressor having a high efficiency can be provided.